1. Field of the Invention
The present invention relates to polymer compositions and methods for making polymer compositions useful as components in nonwoven fabrics. This invention more particularly pertains to extruding a polymer composition into a film and exposing the film to aqueous solvents to remove a soluble polymer phase, resulting in a porous, wettable, polyolefin fabric that is flushable.
2. Description of the Background Art
Disposable personal care products such as diapers, tampons, pantiliners etc. are a great convenience. Such products conveniently provide the benefit of one time, sanitary use and are quick and easy to use. However, disposal of such products is a concern because of depleted landfill space and the undesirability of incineration. Moreover, the difficulty and costs associated with separating such products in preparation of disposal is also of concern. Consequently, there is a need for a porous, wettable, nonwoven fabric which can maintain its intended structure during personal use, but which is entirely acceptable in conventional sewage systems. Personal care products which are flushable in conventional sewage systems provide the benefit of convenient, cost effective and conscientious disposal.
Presently, commercially available fabrics of composite polymers, produced with the use of extrusion devices, are often woven after the polymer composition is forced through the fine holes of a spinneret to form continuous filaments of the man-made fiber. The subsequent drafting and twisting together of the fibers to form the yarn is called spinning. Moreover, various methods of spinning are known such as melt spinning, solution spinning and flash spinning. To make fabric, the fibers are twisted together to make a strand of yarn and, typically, the strands of spun yarn are then woven together.
There are commercially available fabrics of composite polymers which are not woven. These known nonwoven fabrics are produced from processes which are equally known. For example, one of these nonwoven fabrics is known as a spunbond fabric. A spun bond fabric uses a spinneret to form fibers which fall down into an air gun. The fibers are sprayed with air so that the continuous fibers are randomly laid upon one another. The entangled, continuous fibers are then wound onto a forming wire. Since the fibers are not woven, bonding is necessary to allow the entangled fibers to maintain their desired form. Adhesive or heating is often used to bond the entangled fibers to form the fabric.
A second type of nonwoven fabric is meltblown fabric. In this example, the fibers are extruded from a spinneret as with spunbonding, but they are very fine. A fast stream of air is blown into the melt as it exits from the holes of the spinneret. The air draws the melt to produce microfibers typically 3 to 5 microns in diameter. The web is collected on a forming wire. The fibers are hot when they fall into the mass and form some bonding, but normally an additional thermal or adhesive bonding step is performed.
Another example of a nonwoven fabric is air laid fabric which is also produced from a known process. An air laid nonwoven fabric is produced from small, lint-like fibers dropped in the air. A vacuum draws the fibers down to make a random collection of fibers which may also be adhesively or thermally bonded.
The nonwoven fabrics produced from these known processes may be characterized by their fiber lengths, orientation, and the resulting channels or capillaries between the fibers produced by the bonds between the fibers. In the case of nonwoven fabrics designed for fluid management and distribution, the channels provide a place for water to go. Both elongated fibers and channels are desired in order to obtain optimal fluid intake and wicking along with a strong fabric. The spun bond fabrics include long, continuous fibers which form short channels therebetween. In the case of melt blown fabrics, both the fibers and the channels are short. The air laid fabrics have short fibers but moderately lengthened channels. In any case, the optimal configuration of long, continuous fibers with long channels does not exist in any known nonwoven fabric and, moreover, may not be produced by any known process. Therefore, the desired strength as a result of the structural integrity of bonded continuous fibers in conjunction with the increased water intake and wicking obtained from elongated channels does not exist in these known nonwoven fabrics. In short, a disadvantage associated with the typical nonwoven fabrics is that the fibers are not directionally oriented or the channels are obstructed by bonds between the fibers. The nonwovens, in and of themselves, are not wettable unless they are treated or coated with surfactants. However, these surfactants are not permanent on the nonwoven fabrics.
There are some patents known to disclose the processing of polymeric compositions with preparation of a polymer blend by specifically dissolving one polymer component in another. For example, U.S. Pat. No. 3,539,666 to Schirmir discloses a method for producing a nonwoven fabric-like member. A thermoplastic composition comprising a blend of different polymers is extruded through an annular die to form a seamless cellular tube which is biaxially stretched by drawing over a mandrel. A substantial portion of the individual cells in the tube rupture to form a porous web-like structure resembling a nonwoven fabric. Once cooled, the resultant structure may be drawn off the mandrel and slit to form a sheet.
U.S. Pat. No. 5,178,812 to Sanford et al. discloses a method of making composites having improved surface properties. In Sanford, a polymer matrix is extracted from the interior of a composite material in order to increase the matrix concentration at the material's surface. Before selectively dissolving the polymer matrix with an acidic solvent, the fibers may be formed by spinning, extruding a dope into films or fibridating the dope into fibrids. The process in Sanford then involves treating the material with a solvent which dissolves the polymer matrix while not substantially dissolving the reinforcing polymer phase. Next, the solvent is removed whereby at least some of the polymer matrix is extracted from the interior of the material and the matrix concentration increases at the material's surface. In short, Sanford simply teaches a process for improving surface properties such as adhesiveness in a composite construction. Moreover, Sanford does not disclose a process for removing substantial portions of the polymer matrix from the polymer composite as does the present invention.
U.S. Pat. No. 5,096,640 to Brody et al. discloses a method of producing a highly porous, melt spun, fibrous tube for use as a separation medium. In Brody, a blend of polymer components are placed in a solvent. One of the two components is then leached out to produce a tube having a wall consisting of interpenetrating networks of two polymeric components. One of the two components in the interpenetrating network leaches out to produce the tube. Brody requires the blend to be spinnable. Moreover, Brody neither teaches a simple blend nor a permanently wettable fabric as does the present invention.
U.S. Pat. No. 5,227,101 to Mahoney et al. teaches preparation of a porous membrane from a polymer blend by dissolving one component of the blend. In Mahoney, the invention relates to a process for making microporous membranes for liquid or gas separations by mixing a poly(etheretherketone)-type polymer and a low melting point crystallizable polymer. A plasticizer dissolves at least 10 weight percent of the poly(etheretherketone)-type polymer that is present at the membrane fabrication temperature. The plasticizer may be made of a solvent such that at least 10 weight percent of the poly(etheretherketone)-type polymer is dissolved. Mahoney discloses extruding polymeric mixtures into membranes and then immersing the membrane in a leach bath. In Mahoney, the mixture of polymers and plasticizer is extruded through a spinneret. The invention in Mahoney focuses on the manufacture of filtration and/or separation membranes. However, a process for making a nonwoven fabric as in the present invention is not disclosed.
U.S. Pat. No. 3,323,978 to Rasmussen also discloses processing of polymeric composition to form textile fibers. Rasmussen teaches a two-phase fibrous microstructure comprising a distinctly hydrophobic component and a distinctly hydrophilic component. The fibers are produced from a film material which is treated with a swelling agent for the distinctly hydrophilic component. The swelled product is subsequently split into individual fibers or a coherent network of fibers. By swelling, the material of the hydrophilic fibrils is weakened. The surfaces resulting from the splitting will mainly be the in the hydrophilic substance so that the fibers will have an accumulation of hydrophilic substance at the surfaces which is what Rasmussen discloses as desirable. Rasmussen also does not teach a process for making a nonwoven, porous fabric as does the present invention.
Although dissolving a polymer component of a material is known in the art, these known processes are troublesome or outright impractical. Fibers formed from higher molecular weights such as film grade polyethylene cannot easily be formed into fibers even deliberately by the known, conventional spinning processes. For example, melt spinning difficulties arise from the extremely high melt viscosity of the high molecular weight resin at the shear rate typically encountered in melt spinning processes such as melt blown and spunbond processes. Further, the high molecular weight polyethylene has inherently high melt strength and low melt drawability which makes the aerodynamic drawing very difficult.
Thus, despite the attempts described above to produce nonwoven fabrics and to form materials from polymer blends, no method has been developed to produce a polyolefin fabric that is wettable and that can be accepted as flushable through conventional waste water disposal systems.
Due to its unique interaction with water and body fluids, polyethylene oxide (hereinafter PEO) may be utilized as a component material for flushable products. PEO, EQU --(CH.sub.2 CH.sub.2 O).sub.n --,
is a commercially available water-soluble polymer that can be produced from the ring opening polymerization of the ethylene oxide, ##STR1## Because of its water-soluble properties, PEO is desirable for flushable applications. However, there is a dilemma in utilizing PEO in the flushable applications.
Low molecular weight PEO resins have desirable melt viscosity and melt pressure properties for extrusion processing but have limitations when melt processed into structural articles such as thin films. An example of a low molecular weight PEO resin is POLYOX.RTM. WSR N-80 which is commercially available form Union Carbide. POLYOX.RTM. WSR N-80 has an average approximate molecular weight of 200,000 g/mol as determined by melt rheology measurements. As used herein, low molecular weight PEO compositions are defined as PEO compositions with an average molecular weight of less than and including approximately 200,000 g/mol.
In the personal care industry, thin-gauged films are desired for commercial viability and ease of disposal. The low melt strength and low melt elasticity of low molecular weight PEO limit the ability of the low molecular weight PEO to be drawn into films having a thickness of less than about 2 mil. Although low molecular weight PEO can be thermally processed into films, thin-gauged films of less than about 1 mil in thickness cannot be obtained due to the lack of melt strength and melt elasticity of the low molecular weight PEO. The processability of PEO may be improved by blending the PEO with a second polymer, a copolymer of ethylene and acrylic acid, in order to increase the melt strength. The PEO/ethylene acrylic acid copolymer blend may be processed into films of about 1.2 mils in thickness. However, the blend and resulting film are not water-soluble. More importantly, thin films made from low molecular weight PEO are too weak and brittle to be useful for personal care applications. Low molecular weight PEO films have low tensile strength, low ductility, and are too brittle for commercial use. Further, films produced from low molecular weight PEO and blends containing low molecular weight PEO become brittle during storage at ambient conditions. Such films shatter and are not suited for commercial applications.
High molecular weight PEO resins are expected to produce films with improved mechanical properties compared to films produced from low molecular weight PEO. An example of a high molecular weight PEO is POLYOX.RTM. WSR 12K which is commercially available from Union Carbide. POLYOX.RTM. WSR 12K has an average approximate molecular weight of 1,000,000 g/mol as determined by melt rheology measurements. As used herein, high molecular weight PEO compositions are defined as PEO compositions with an average molecular weight of greater than and including approximately 300,000 g/mol.
High molecular weight PEO has poor processability due to its high melt viscosity and poor melt drawability. Melt pressure and melt temperature are significantly elevated during melt extrusion of high molecular weight PEO. During extrusion of high molecular weight PEO, severe melt fracture is observed. Only very thick sheets can be made from high molecular weight PEO. High molecular weight PEO cannot be thermally processed into films of less than about 7 mil in thickness. High molecular weight PEO suffers from severe melt degradation during extrusion processes. This results in breakdown of the PEO molecules and formation of bubbles in the extrudate. The inherent deficiencies of high molecular weight PEO makes it impossible to utilize high molecular weight PEO in film applications. Even the addition of high levels of plasticizer to the high molecular weight PEO do not improve the melt processability of high molecular weight PEO sufficiently to allow the production of thin films without melt fracture and film breakage occurring.
Therefore, currently available PEO resins are not practical for melt extrusion, for thin films, or for personal care applications. What is needed in the art, therefore, is a means to overcome the difficulties in melt processing of high molecular weight PEO resins and the insufficiencies of thin films thermally processed from currently available low molecular weight PEO resins.
Thus, there has been a need in the art for a process for making a nonwoven polyolefin fabric that is produced without spinning or drawing from polymer components of varying molecular weight, is strong enough for extended personal use, has enhanced fluid intake and wicking, may be permanently wettable, and is entirely flushable down conventional sewage systems.